A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate.
During lithographic operation, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it may be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks, which may comprise diffraction gratings are placed on the substrate to be aligned and are located with reference to a second object. Lithographic apparatus may use an alignment system for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask.
Alignment systems typically have their own illumination system that may be used to illuminate the alignment marks during alignment measurements. The radiation diffracted from the illuminated alignment marks may be detected and used to determine the positions of the alignment marks. The accuracy and characteristics of diffraction signals detected from the illuminated alignment marks may be dependent on the polarization state of the radiation beam used to illuminate the alignment marks. This dependency is due to the diffraction efficiency of the alignment marks being related to the polarization state of the radiation beam.
The radiation beam may have linear, circular, or elliptical polarization states. Each polarization state can be characterized by two orthogonal polarization states. For example, linear polarization state can be characterized by horizontal and vertical polarization states, and circular polarization can be characterized by right- and left-handed polarization states. A horizontally or vertically polarized radiation beam may be used for alignment marks composed of either vertical or horizontal lines. The polarization can be parallel or perpendicular to the vertical or horizontal lines. A circularly polarized radiation beam may be useful for alignment marks having lines or gratings of both horizontal and vertical orientations or having gratings with unknown orientations. A circularly polarized radiation beam can be considered to have two perpendicular polarized components of equal amplitude that are either +90° or −90° out of phase with respect to each other and can be characterized as right- and left-handed polarization states based on the rotation of its components. Either or both of the two circular polarization states may be used for alignment measurements.
To provide circularly polarized radiation beams for alignment measurements, some of the current alignment systems use, for example, quarter wave plates to convert linearly polarized radiation beams from the illuminations system into circularly polarized radiation beams. These circularly polarized radiation beams are then directed onto the alignment marks using fold mirrors. In some alignment systems, the fold mirror may contain a metallic layer. A disadvantage of using such fold mirrors is that at least a portion of the radiation beam is absorbed by the fold mirrors, while being directed onto the alignment marks. Such loss of radiation through absorption may cause, for example, reduction in diffraction efficiency of the alignment marks and reduction in diffraction signal intensity detected from the alignment marks. Another disadvantage is that the fold mirrors become heated during operation due to the absorption of at least a portion of the radiation beam. Such heating of the fold mirrors may cause, for example, a wavefront error in the radiation beam reflected from the alignment marks, which may result in inaccurate alignment measurements of the alignment system.